In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least two layers: a photoconductive layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of photoconductive materials which have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanine. Electrophotographic recording elements containing such pigments as charge-generation materials are useful in electrophotographic laser beam printers because they are capable of providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is in the range of 700-900 nm.
The photoconductive layer is produced from a liquid coating composition which includes the titanyl phthalocyanine pigment and a solvent solution of polymeric binder. It is necessary that the titanyl phthalocyanine pigment be in a form, either crystalline or amorphous, that is highly photoconductive and sufficiently and stably dispersed in the coating composition to permit its being applied at a low enough concentration to form a very thin layer having high electrophotographic speed in the near infrared range. A variety of methods have been used to produce suitable forms of titanyl phthalocyanine. Different methods have commonly produced titanyl phthalocyanines having differing crystallographic characteristics. (U.S. Pat. No. 5,166,339 to Duff et al presents a table of polymorphs of unsubstituted titanyl phthalocyanine (also referred to as "TiOPc") in which materials bearing multiple designations are grouped as four types. Many types of TiOPc and other phthalocyanines are discussed in Organic Photoreceptors for Imaging Systems, P. M. Borsenberger and D. S. Weiss, Marcel Dekkar, Inc., New York, pp. 338-391.)
In one group of preparations, commonly referred to as "acid-pasting", crude titanyl phthalocyanine is dissolved in an acid solution, which is then diluted with non-solvent to precipitate the titanyl phthalocyanine product. In another group of preparations, the crude titanyl phthalocyanine is milled, generally with particular milling media. Some preparations combine techniques or modify a previously prepared titanyl phthalocyanine.
U.S. Pat. No. 5,132,197 to Iuchi et al teaches a method in which titanyl phthalocyanine was acid pasted, treated with methanol, and milled with ether, monoterpene hydrocarbon, or liquid paraffin to produce a titanyl phthalocyanine having main peaks of the Bragg angle 2.theta. with respect to X-rays of Cu Ka at 9.0.degree., 14.2.degree., 23.9.degree., and 27.1.degree. (all +/-0.2.degree.).
U.S. Pat. No. 5,206,359 to Mayo et al teaches a process in which titanyl phthalocyanine produced by acid pasting is converted to type IV titanyl phthalocyanine from Type X by treatment with halobenzene.
U.S. Pat. No. 5,059,355 to Ono et al teaches a process in which TiOPc was shaken with glass beads producing an amorphous material having no substantial peaks by X-ray diffraction. The amorphous material was stirred with heating in water and ortho-dichlorobenzene. Methanol was added after cooling. A crystalline material was produced which had a distinct peak at 27.3 degrees.
U.S. Pat. No. 4,882,427 to Enokida et al teaches a material having noncrystalline titanyl phthalocyanine and pseudo-non-crystalline titanyl phthalocyanine. The pseudo-noncrystalline material could be prepared by acid pasting or acid slurrying. The noncrystalline titanyl phthalocyanine could be prepared by acid pasting or acid slurrying followed by dry or wet milling, or by mechanical milling for a long time without chemical treatment.
U.S. Pat. No. 5,194,354 to Takai et al teaches that amorphous titanyl phthalocyanine prepared by dry pulverization or acid pasting can be converted, by stirring in methanol, to a low crystalline titanyl phthalocyanine having strong peaks of the Bragg angle 2-theta with respect to X-rays of Cu Ka at 7.2.degree., 14.2.degree., 24.0.degree. and 27.2.degree. (all +/-0.2.degree.). The low crystalline material, it was indicated, could be treated with various organic solvents to produce crystalline materials: methyl cellosolve or ethylene glycol for material having strong peaks at 7.4.degree., 10.9.degree., and 17.9.degree.; propylene glycol, 1,3-butanediol, or glycerine for material having strong peaks at 7.6.degree., 9.7.degree., 12.7.degree., 16.2.degree., and 26.4.degree.; and aqueous mannitol solution for material having strong peaks at 8.5.degree. and 10.2.degree. (all peaks +/-0.2.degree.).
U.S. Pat. No. 4,994,566 and 5,008,173, to Mimura et al teach a process in which non-crystalline particles produced by: acid pasting or slurrying then mechanical grinding, mechanical grinding for a very long time, or sublimation; are treated with tetrahydrofuran to produce the a titanyl phthalocyanine having infrared absorption peaks at 1,332; 1,074; 962; and 783 cm.sup.-1.
U.S. Pat. No. 5,039,586 to Itami teaches acid pasting followed by milling in aromatic or haloaromatic solvent with or without additional water or other solvents such as alcohols or ethers, at 20.degree.-100.degree. C. In an example, crude titanyl phthalocyanine was milled with alpha-chloronaphthalene or ortho-dichlorobenzene as milling medium followed by washing with acetone and methanol. The titanyl phthalocyanine produced had a first maximum intensity peak of the Bragg angle 2-theta with respect to X-rays of Cu Ka at a wavelength of 1.541 .ANG. at 27.3.degree.+/-0.2.degree., and a second maximum intensity peak at 6.8.degree.+/-0.2.degree.. This was contrasted with another titanyl phthalocyanine which was similarly milled, but not acid pasted. This material had a maximum peak at 27.3.degree.+/-0.2.degree. and a second maximum intensity peak, in the 6-8.degree. range, at 7.5.degree. +/-0.2.degree..
U.S. Pat. No. 5,055,368, to Nguyen et al teaches a "salt-milling" procedure in which crude titanyl phthalocyanine is milled, first under moderate shearing conditions, along with milling media comprising inorganic salt and non-conducting particles. The milling is then continued at higher shear and temperatures up to 50.degree. C. until the pigment undergoes a perceptible color change. Solvent is substantially absent during the milling steps.
U.S. Pat. 4,701,396, to Hung et al, teaches near infrared sensitive photoconductive elements made from fluorine-substituted titanylphthalocyanine pigments. While phthalocyanines having only fluorine substituents, and those being equal in number on each aromatic ring, are the preferred pigments of the invention described in that patent, various non-uniformly substituted phthalocyanines are taught.
U.S. Pat. Nos. 5,238,764 and 5,238,766, both to Molaire et al, teach that titanyl fluorophthalocyanine products of acid-pasting and salt-milling procedures, unlike unsubstituted titanyl phthalocyanine, suffer a significant reduction in near infrared sensitivity when they are dispersed in a solvent such as methanol or tetrahydrofuran, which has a gamma.sub.c hydrogen bonding parameter value greater than 9.0. These patents further teach that this reduction in sensitivity can be prevented by first contacting the titanyl fluorophthalocyanine with a material having a gamma.sub.c hydrogen bonding parameter of less than 8.0.
U.S. Pat. No. 5,112,711 to Nguyen et al teaches an electrophotographic element having a physical mixture of titanyl phthalocyanine crystals and titanyl fluorophthalocyanine crystals. The element provided a synergistic increase in photosensitivity in comparison to an expected additive combination of titanyl phthalocyanine and titanyl fluorophthalocyanine. Similar elements having physical mixtures combining titanyl phthalocyanine crystals and chloroor bromo-substituted titanyl phthalocyanine crystals produced results in which the photosensitivity was close to that of the least sensitive of the two phthalocyanines used.
The above procedures present shortcomings in the preparation of titanyl phthalocyanines. The use of acid presents a shortcoming for both environmental and safety concerns, particularly in commercial scale procedures. Salt milling avoids the use of acid, but requires extensive washing of the milled material to remove salts, which can otherwise cause high dark decay in a photoconductor. Procedures which first contact the titanyl fluorophthalocyanine with a solvent such as methanol or tetrahydrofuran, having a gamma.sub.c hydrogen bonding parameter value greater than 9.0 cause a significant reduction in near infrared sensitivity. Appropriately prepared titanyl fluorophthalocyanine has good photogeneration characteristics, but is expensive.
It is therefore desirable to provide a method for producing a crystalline titanyl phthalocyanine composition which has good photogeneration characteristics when used in an electrophotgraphic element, but is less expensive than titanyl fluorophthalocyanine. It is further desirable that the method avoid deleterious contact with high gamma.sub.c hydrogen bonding parameter solvents, and not require the use of acid or of salt milling media.